JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011, P. 907
Enhancing the performance of photovoltaic cells by using down-converting
KCaGd(PO4)2:Eu3+ phosphors
Yen-Chi Chen (陈彦吉), Woan-Yu Huang (黄琬妤), Teng-Ming Chen (陈登铭)
(Phosphors Research Laboratory and Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, China)
Received 8 April 2011; revised 24 May 2011
Abstract: The goal of this work is aimed to improve the power conversion efficiency of single crystalline silicon-based photovoltaic (PV)
cells by using the solar spectral conversion principle, which employed a down-converting phosphor to convert a high-energy ultraviolet photon to the less energetic red-emitting photons to improve the spectral response of Si solar cells. In this study, the surface of silicon solar cells
was coated with a red-emitting KCaGd(PO4)2:Eu3+ phosphor by using the screen-printing technique. In addition to the investigation on the microstructure using scanning electron microscopy (SEM), we measured the short circuit current (Isc), open circuit voltage (Voc), and power conversion efficiency (η) of spectral-conversion cells and compared with those of bare solar cells as a reference. Preliminary experimental results
revealed that in an optimized PV cell, an enhancement of (0.64+0.01)% (from 16.03% to 16.67%) in Δη of a Si-based PV cell was achieved.
Keywords: solar cells; down-converting phosphor; KCaGd(PO4)2:Eu3+; screen-printing; rare earths
To face the challenge of global warming, the development
of green energy materials has been an important issue in
materials research. The photovoltaic (PV) cell is one of the
devices that can be used to generate sustainable energy;
therefore, many research attempts have been made to explore materials that are able to enhance the power conversion
efficiency (η) of solar cells. The conversion efficiency from
light to electricity in a PV cell is highly dependent on the
wavelength (λ) of incident light, and the η-λ relationship is
characterized by the spectral response. In general, the PV
cells are able to convert only a small portion (i.e., longer
wavelength domain) of solar spectrum into electricity, with
ultraviolet (UV) and infrared (IR) spectral domains wasted.
Attempts to improve the conversion efficiency of PV cells
using spectral conversion technique by employing up- or
down-conversion phosphors have been well documented in
literature[l–7].
In recent years, a series of double phosphates, represented
as ABM(PO4)2 (where A=alkali metal, B=alkaline earth, M=
Gd, Y or La) and isotypic with LaPO4, have been reported[8–14]. With appropriate doping, the ABM(PO4)2 phosphates were reported to show wide applications in plasma
displays and mercury-free lamps, mainly because the host of
ABM(PO4)2 exhibits low phonon energy and potential
quantum-cutting property[14].
In this research, the effect of coating a down-conversion
phosphor that is expected to form a radiation-sensitive surface on the silicon-based PV cells in attempt to increase the
power conversion efficiency was investigated. Essentially,
this deposited phosphor layer is suitable for absorption and
emission in the portion of solar spectrum and further benefits
utilization of sun light, thus improving the η value of the PV
cells. Furthermore, the coated phosphor layer exhibits lower
refractive index, which may serve as an antireflection layer
in addition to solar spectral conversion[15].
We have screened and selected phosphors such as
KCaGd(PO4)2:Eu3+ (KCGP:Eu3+), with low phonon energy
and lower refractive index than Si or Si3N4 that may be capable of converting more UV photons into photons with
longer wavelength to induce a greater spectral response for a
Si-based PV cell. This work attempted to evaluate and examine the potential applications of the down-converting
KCaGd(PO4)2:Eu3+ in an attempt to improve the efficiency
of silicon-based PV cells. We utilized a screen-printing
technique to form a phosphor layer directly onto a commercial Si PV cell to convert the UV photons into those
with wavelength longer than 500 nm. The dependence of
photovoltaic efficiency on the phosphor compositions,
photoluminescence (PL) and PL excitation (PLE) spectra,
and microstructure of the phosphor layer were investigated
and discussed.
1 Experimental
Stoichiometric starting materials of (NH4)2HPO4, K2CO3,
Eu2O3, SrCO3, CaCO3 (all analytic grade), and Gd2O3
(99.99% pure) were mixed together with NH4Cl as a flux
and transferred to an alumina crucible; the materials were
then heat treated at 800 °C for 6 h and at 1200 °C for 6 h. In
comparison with the process described by Zhang et al.[14],
our synthesis process takes only one-third of the time needed
Foundation item: Project supported by National Science Council of Taiwan (NSC98-2113-M-009-005-MY3)
Corresponding author: Teng-Ming Chen (E-mail: [email protected]; Tel.: +886-3-5731695)
DOI: 10.1016/S1002-0721(10)60565-0
908
to prepare KMGd(PO4)2 (M=Ca, Sr). The phase purity of
KMGd(PO4)2:Eu3+ phosphors was checked by powder X-ray
diffraction with a Bruker AXS D8 advanced automatic diffractometer with Cu Kα radiation and all of reflections between 2θ=10° and 80° were collected at room temperature.
Photoluminescence (PL) and PL excitation (PLE) spectra
were obtained using a Jobin Yvon-Spex FluoroLog-3
fluorophotometer equipped with a 450 W Xe lamp as light
source.
The fabrication of KCaGd(PO4)2:Eu3+ (KGP:Eu3+)-coated
solar cells is summarized in the flow diagram shown in Fig. 1.
Briefly, the KCaGd(PO4)2:Eu3+ phosphor was well-mixed
and dispersed in a composite polymethylmethacrylate
(PMMA) polymer binder and the phosphor/binder mixture
was then screen-printed on top of the prestructured Si3N4 reflective layer of a 6”×6” Si solar cell to form a transluscent
film with 3–4 μm in thickness. The phosphor-coated solar
cell was then baked at about 130 ºC in the air for 10 min.
The device structure of down-converting KCaGd(PO4)2:Eu3+
phosphor-coated solar cells is schematically shown in Fig. 2.
Furthermore, the open-circuit voltage (Voc), short-circuit
current (Isc), and power conversion efficiency (η) of the
phosphor-coated solar cells were then measured using an
h.a.l.m IV curve tracer (cetisPV-CTL1) and a sun simulator
(Xenon-Flasher cetisPV-XF2).
2 Results and discussion
The XRD patterns of KCaGd(PO 4 ) 2 :Eu 3 + and
KSrGd(PO4)2:Eu3+ samples shown in Figs. 3(1) and (2), respectively, were found to match well with those reported in
JCPDS cards 34-0125 and 34-0118, respectively. Except for
slight differences in the cell parameters of the unit cell,
JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011
Fig. 3 Indexed XRD patterns of KCaGd(PO4)2:Eu3+ (1) and
KSrGd(PO4)2:Eu3+ (2)
KCaGd(PO4)2 and KSrGd(PO4)2 have the same crystal
structure similar to KCaNd(PO4)2, which is isostructural with
hexagonal LaPO4.
The KCaGd(PO4)2 host was found not to absorb in the ultraviolet region. The KGP:Eu3+ phosphor can be excited with
393 nm UV light and produces orange-red emission peaking
at 585 nm. The PLE spectrum of KGP:Eu3+ phosphor shows
absorption in the wavelength domain of 260 to 530 nm and a
maximal emission in the wavelength domain of 580 to 700
nm, as indicated in Fig. 4. Since the silicon wafer shows
poor absorption in the UV spectral range, applying the
KGP:Eu3+ or KSP:Eu3+ phosphor as a radiation-sensitive
layer on the surface a PV cell would be expected to increase
the UV absorption and thus enhance the efficiency of power
conversion.
Fig. 5 shows and compares the reflectance spectra for bare,
solely binder-coated, and KGP:xEu3+ (x=5%, 10%, 30%,
50%, and 100%) -coated solar cells.
Comparison of the reflectance spectra for the bare, bindercoated, and phosphor-coated cells indicates that the phosphor coating on the surface of the Si wafer can effectively
reduce reflection and increase light absorption. A drastic reduction in the reflectance was observed for the phosphor
KCGP:100%Eu3+-coated cell. In addition to the decrease in
reflectance, the observed Isc obtained from KCGP:Eu3+coated solar cells was simultaneously found to increase with
phosphor coating. Fig. 6 presents the current-voltage data for
an optimized Si solar cells with and without coating of
Fig. 1 Flow diagram for fabrication of phosphor-coated Si solar
cells
Fig. 2 Device structure of down-converting KCaGd(PO4)2:Eu3+
phosphor-coated solar cells
Fig. 4 PLE and PL spectra of KCaGd(PO4)2 at room temperature
Yen-Chi Chen et al., Enhancing the performance of photovoltaic cells by using down-converting KCaGd(PO4)2:Eu3+ phosphors
Fig. 5 Comparison of reflectance spectra for bare and KGP:Eu3+coated solar cells (From top: bare cell, binder+cell, and cells
with 5%, 10%, 30%, 50%, and 100% KGP:Eu3+ coating)
KCGP:Eu3+, respectively. Data analysis indicates that
KCGP:Eu3+-coated solar cell has greater Isc due to the efficient light conversion. That is, the Isc value was found to increase from 7.9664 to 8.3058 A, Voc increases from 0.6231
to 0.6247 V, and η was found to increase from 16.03% to
16.67%. These data revealed that Isc increases significantly
with Voc unchanged upon phosphor coating.
To further verify the experimental results that coating of
KCGP:Eu3+ phosphor increases the power conversion efficiency, we measured the Isc, Voc, η, Isc, Voc and η for forty solar cells coated with and without KCGP:Eu3+ phosphor. Table 1 summarizes the comparison on the average values and
increases appreciably from 8.10 to 8.35 A and the
open-circuit voltage varies from 0.63115 to 0.63200 V in-
909
standard deviations of Isc, Voc, η, Isc, Voc, and η.
On average, we have observed that short-circuit current
significantly, respectively.
We have also observed that η increases for 0.48% from an
average value of 16.52% to 17.00%. To investigate the microstructure of the phosphor-coated solar cell, we have investigated the SEM micrographs of a screen-printed
KGP:Eu3+- coated solar cell after baking. Figs. 7(a) and (b)
show the top- and side-view of the cell and the surface exhibits granular feature inherited from phosphor particles,
whereas the radiation-sensitive layer is estimated to be 3.4
μm in thickness, as revealed in the side-view SEM micrograph.
Table 1 Comparison of averaged Isc, Voc, η, Isc, Voc, and η obtained for forty solar cells with and without coating of
KCaGd(PO4)2:Eu3+ investigated in this work
Isc(1) Isc(2) ΔIsc Voc (1) Voc(2) ΔVoc η1
Δη (η2−η1)
η2
Average 8.10 8.35 0.25 0.63115 0.63200 0.0085 16.52% 17.00% 0.48 %
Standard
0.02 0.02
0.0010 0.0010
0.0495 0.0517
deviation
Fig. 7 SEM micrographs of a KGP:Eu3+-coated Si solar cell
(a) Top view; (b) Side view
3 Conclusions
Fig. 6 Experimental current-voltage curves for a representative Si
solar cells without (a) and with KGP:Eu3+ phosphor coating (b)
We prepared a double phosphate phosphor KCaGd(PO4)2:
Eu3+ and demonstrated that the down-converting
KCaGd(PO4)2:Eu3+ phosphor coated on the surface of a
polycrystalline silicon solar cell could effectively increase
the values of Isc, Voc, and η of the cell. The increase in η was
about 0.48% on the average, which corresponded to an increment from 16.52% to 17.00%. However, in an optimized
case, we observed an increase in η from 16.03% to 16.67%,
which corresponded to an increase of 0.64%. Coating orange
910
red-emitting down-converting phosphors by screen printing
technique on the surface of conventional solar devices was
effective in enhancing the η value and was demonstrated in
this research. The coated phosphor formed not only a spectral conversion layer for one part of the solar spectrum but
also served a low reflective layer for a different part of the
solar spectrum. The PMMA might provide a transparent matrix for phosphor coating. Further work to improve the
power conversion efficiency and select more efficient phosphors for solar application is currently in progress.
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